Don't listen to these people. They do it the old fashioned way, for the Tour is nothing if not about tradition. They send a low level pro up the climbs. They tell him to ride it "really hard". At the top, they ask him how it was. They then count how many sentences the pro says in a specific period of time. (sorry, can't remember the time). 4 is 4 sentences, 3, three, and down to one where the guy can just get out a complete sentence. The original scale only had 4 to 1 ratings. But one time, the rider just gasped out, "hors of a climb" before he fell on the ground. That is a fragment, not a sentence. The rating scale that was 4 to 1 had to have a new category, HC. There is a lesson about scale construction in there you know.

All the other reasons for categorizing are propaganda, to hide the French love of torturing low level pros.